The present invention relates to a one-stage anodic oxidation process for aluminum which is employed as a base for offset printing plates, the base resulting from this process and the offset printing plate itself.
Bases for offset printing plates are provided, either directly by the user or by the manufacturer of precoated printing plates, with a radiation-sensitive or photosensitive layer (reproduction layer) on one or both sides, with the aid of which layer a printable image is produced by photomechanical means. After production of a printing form from the printing plate, the base carries the image areas which convey ink during subsequent printing and, in the areas which are image-free during subsequent printing (non-image areas), also forms the hydrophilic image background for the lithographic printing process.
Bases for reproduction layers for the production of offset printing plates therefore have to meet the following requirements:
The areas of the radiation-sensitive layer which are relatively more soluble after exposure must be capable of being readily removed from the base without leaving a residue to produce the hydrophilic non-image areas, this being done without the developer attacking the base to any great extent. PA1 The base bared in the non-image areas must have a great affinity for water, i.e., must be very hydrophilic, in order, in the lithographic printing process, to take up water rapidly and permanently and to have a sufficiently repellent action toward the fatty printing ink. PA1 The adhesion of the photosensitive layer before exposure, and of the printing areas of the layer after exposure, must be adequate. PA1 The base should possess good mechanical stability, for example to abrasion, and good chemical resistance, in particular to alkaline media.
A particularly frequently used starting material for such bases is aluminum, the surface of which is roughened by conventional methods, for example, by dry-brushing, wet-brushing, sand blasting, chemical treatment and/or electrochemical treatment. To increase the abrasion-resistance, electrochemically roughened substrates, in particular, are subjected to an anodizing step to build up a thin oxide layer. These anodic oxidation processes are usually carried out in electrolytes such as H.sub.2 SO.sub.4, H.sub.3 PO.sub.4, H.sub.2 C.sub.2 O.sub.4, H.sub.3 BO.sub.3, amidosulfonic acid, sulfosuccinic acid, sulfosalicylic acid or mixtures of these. The oxide layers produced in these electrolytes or mixtures of electrolytes differ in structure, layer thickness and resistance to chemicals. In practice, in offset printing plate production, an aqueous H.sub.2 SO.sub.4 or H.sub.3 PO.sub.4 solution is particularly employed. With regard to H.sub.2 SO.sub.4 -containing electrolytes, reference may be made to, for example, U.S. Pat. No. 4,211,619 and the prior art mentioned therein.
Aluminum oxide layers produced in aqueous H.sub.2 SO.sub.4 -containing electrolytes are amorphous and, when used in offset printing plates, usually have a weight per unit area of about 0.5 to 10 g/m.sup.2, corresponding to a layer thickness of about 0.15 to 3.0 .mu.m. The disadvantage of using such an anodically oxidized base for offset printing plates is the fact that the oxide layers produced in H.sub.2 SO.sub.4 electrolytes have a relatively low resistance to alkaline solutions as used to an increasing extent in, for example, the processing of presensitized offset printing plates, preferably in modern developer solutions for irradiated negative-working or, in particular, positive-working radiation-sensitive layers.
The anodic oxidation of aluminum in aqueous electrolytes containing phosphorus oxyacids or phosphates is likewise known per se:
U.S. Pat. No. 3,511,661 describes a process for the production of a lithographic printing plate, in which the aluminum base is oxidized anodically at a temperature of at least 17.degree. C. in an at least 10% strength aqueous H.sub.3 PO.sub.4 solution, until the aluminum oxide layer has a thickness of at least 50 nm.
U.S. Pat. No. 3,594,289 discloses a process in which a printing plate base made of aluminum is oxidized anodically in a 50% strength aqueous H.sub.3 PO.sub.4 solution at a current density of 0.5 to 2.0 A/dm.sup.2 and at a temperature of 15.degree. to 40.degree. C.
The process for the anodic oxidation of aluminum bases, in particular for printing plates, according to U.S. Pat. No. 3,836,437 is carried out in a 5 to 50% strength aqueous Na.sub.3 PO.sub.4 solution at a temperature of 20.degree. to 40.degree. C. and a current density of 0.8 to 3.0 A/dm.sup.2 and for a period of 3 to 10 minutes. The aluminum oxide layer thus produced should have a weight of 10 to 200 mg/m.sup.2. The aluminum can also be mechanically or chemically roughened or etched beforehand.
The aqueous bath for the electrolytic treatment of aluminum which is to be subsequently coated with a water-soluble or water-dispersible substance contains, according to U.S. Pat. No. 3,960,676, 5 to 45% of silicates, 1 to 2.5% of permanganates, or borates, phosphates, chromates, molybdates or vanadates in an amount from 1% to saturation. Preparation of bases for printing plates is not mentioned, nor is prior roughening of the material.
British Pat. No. 1,587,260 discloses a base for printing plates which carries an oxide layer which is produced by anodic oxidation of aluminum in an aqueous solution of H.sub.3 PO.sub.3 or a mixture of H.sub.2 SO.sub.4 and H.sub.3 PO.sub.3. This relatively porous oxide layer is then covered with a second oxide film of the "barrier layer" type, which can be formed, for example, by anodic oxidation in aqueous solutions containing boric acid, tartaric acid or borates. Both the first stage (Example 3, 5 min) and the second stage (Example 3, 2 min) are carried out very slowly, and furthermore, the second stage is carried out at a relatively high temperature (80.degree.).
It is true that an oxide layer produced in H.sub.3 PO.sub.4 is often more resistant to alkaline media than is an oxide layer produced in an electrolyte based on H.sub.2 SO.sub.4 solution. This oxide layer while having some other advantages, such as a paler surface, better water/ink balance or less adsorption of dyes ("staining") in the non-image areas), also possesses significant disadvantages. In a modern conveyor line for the production of printing plate bases, it is possible, using voltages and residence times conforming to practice, to produce oxide layers having a weight per unit area of, for example, only up to about 1.5 g/m.sup.2, which corresponds to a layer thickness which of course provides less protection from mechanical abrasion than does a thicker oxide layer produced in an H.sub.2 SO.sub.4 electrolyte. Because of the relatively large pore volume and pore diameter of an oxide layer produced in H.sub.3 PO.sub.4, the mechanical stability of the oxide itself is lower, and this results in a further loss with respect to abrasion-resistance. There can also be problems of adhesion in certain negative-working layers, so that a printing plate base anodized in H.sub.3 PO.sub.4 cannot be employed in all cases. The prior art oxide layers produced in an aqueous electrolyte containing Na.sub.3 PO.sub.4 require, on the other hand, a treatment time which is too long for a modern high-speed manufacturing line and, by having a weight per unit area of up to only 200 mg/m.sup.2, are furthermore unsuitable for protecting the fine pore structure of an electrochemically roughened aluminum surface sufficiently against mechanical abrasion. For this purpose, weights per unit area of more than 500 mg/m.sup.2, in particular of more than 800 mg/m.sup.2, are required for the high-performance printing plates demanded commercially today.
Processes have also been proposed which seek to combine the advantages of two different electrolytes by employing a two-stage treatment procedures; this also applies to the use of solutions containing phosphate ions in one of the two stages.
German Offenlegungsschift No. 32 06 470, which has not been previously published and has an earlier priority date, describes a two-stage oxidation process for the production of bases for offset printing plates, in which the anodic oxidation is carried out in (a) an aqueous electrolyte based on sulfuric acid and (b) an aqueous electrolyte containing phosphoroxo, phosphorfluoro and/or phosphoroxofluoro anions. A reversed sequence of the oxidation stages is described in a patent application filed concurrently herewith and corresponding to German patent application No. P 33 28 048 and entitled "Process for the Two-Stage Anodic Oxidation of Aluminum Bases for Offset Printing Plates." These two-stage oxidation processes can result in bases, for offset printing plates, which are usable and good in practice and which have the same or similar alkali resistance of an oxide produced in the H.sub.3 PO.sub.4 -containing aqueous electrolytes. However, the bases still require more expensive apparatus since anodic oxidation must be carried out in two baths, frequently also with the intermediate use of a rinsing bath. Such a plant then requires additional units and monitoring procedures, as a result of which, inter alia, additional sources of error can arise.